This invention relates to the preparation of block copolysiloxanecarbonates, and more particularly to their preparation by an improved transesterification method.
The preparation of polycarbonates is typically by one of two methods: the interfacial and the transesterification or melt method. The interfacial method comprises the reaction of at least one dihydroxy compound, generally a dihydroxyaromatic compound, with phosgene in a mixed aqueous-organic (e.g., methylene chloride) medium, in the presence of a basic reagent as acid acceptor and an amine as catalyst.
By reason of the high toxicity of phosgene and the environmental unsuitability of solvents such as methylene chloride, the transesterification or melt method for polycarbonate production is becoming more common. In this method, the dihydroxy compound undergoes reaction in the melt with a diaryl carbonate such as diphenyl carbonate. Initial stages are conducted at temperatures typically up to about 200.degree. C. to produce oligomeric polycarbonate, and the molecular weight is then built by heating at temperatures on the order of 300.degree. C.
Block copolysiloxanecarbonates are used in place of polycarbonates for certain applications, particularly those in which flame retardancy or hydrolytic stability is required and relatively low temperatures of use are intended. A method for their preparation, described in U.S. Pat. No. 3,419,634, comprises the hydrosilylation of an ethylenically unsaturated organic hydroxy compound such as 2-allylphenol with a polydiorganosiloxane containing Si--H moieties, preferably on the terminal silicon atoms, to form a hydroxyorgano-terminated polydiorganosiloxane, and the phosgenation of a mixture of said polydiorganosiloxane and a dihydroxyaromatic compound such as 2,2-bis(4-hydroxyphenyl)propane or "bisphenol A".
The use of a melt procedure for preparation of the copolysiloxanecarbonate is also mentioned in said patent. However, it would not be generally expected that a melt procedure would be suitable since polydiorganosiloxanes usually decompose when heated to the high temperatures required for melt polycarbonate preparation.
In fact, it has been discovered that only low incorporation of polydiorganosiloxane blocks into the polycarbonate is achieved by reaction of diaryl carbonate with a mixture of a dihydroxyaromatic compound and a hydroxyorgano-terminated silicone. This is demonstrated by the fact that a two-phase system is formed upon dissolution of the product in chloroform, the lower phase being predominantly polycarbonate and the upper predominantly polydiorganosiloxane.
There is also a tendency with melt preparation to the formation of copolymers (to the extent they are formed at all) having very high molecular weight polydiorganosiloxane blocks, as a result of incorporation of several such blocks at a single point in the polymer. This can cause haziness in the product, as opposed to a more random distribution of the polydiorganosiloxane blocks which results in the formation of a transparent polymer.
The present invention is based on the discovery of an improved method for preparation block copolysiloxanecarbonates in a melt procedure. Among the advantages which may be observed upon performance of this method are increased incorporation of polydiorganosiloxane and increased randomness of the distribution of the polydiorganosiloxane blocks in the product.